420 N evj combe.— On the Cause and Conditions 
schizogenous factor , if present , is always small, the lysigenous 
factor always large . 
It need probably not be stated that there are many plants 
which begin the formation of cavity in primary growth and 
continue it over into secondary growth. This relation passes 
insensibly into that given in the next statement. 
5. There are species of plants in which some individuals 
form cavities in the pith during primary growth , others during 
secondary growth , the one condition or the other obtaining 
according to the cessation of growth in the pith during or 
subsequently to primary extension in the more peripheral zones 
of tissue. 
6 . The formation of a lysigenous cavity during primary 
growth may be somewhat deferred by preventing the normal 
primary extension of peripheral tissues. 
Asa corollary, it may be added that the tension of tissues is 
a factor in limiting the life-period of cells which normally 
collapse in cavity -formation. 
The evidence for this conclusion is. that in the leaves of 
Allium Cepa, kept from inflating by etiolation or by a gypsum- 
cast, the central cells live longer than normally ; that the 
inner cells of J uncus ejfusus live longer when the shoots 
cannot expand ; that the intercarinal canals of Equisetum 
limosum are deferred in formation when the elongation of 
the internode is prevented ; that the canal in the bundles of 
Zea Mais is deferred in formation when the internode is not 
allowed to elongate ; that several dicotyledonous plants 
preserve the vitality of their pith longer than normally when 
gypsum-casts are put around the stems so as to lessen the 
outward pull on the pith, but not to cause a great constriction 
during the period of primary growth; that no experiments 
or observations have controverted this conclusion. 
7. The formation of cavity during primaiy and secondary 
growth may be greatly deferred by preventhig the extension of 
surrounding tissue. But in this case the residt is to be referred 
to a regtdatory process , probably to the use of the tissue con- 
cerned for purposes of transport. 
